In 2014, Apple unveiled a pair of larger iPhone 6 models that kicked off a "supercycle" of upgrades and permanently blunted the high-end Galaxy sales of its top rival in premium smartphones, Samsung. However, it appears that iPhone 6 and 6S suffered statically high hardware failure rates in diagnostic testing, a problem that has since subsided in more recent models. What caused this mysterious problem, and how did Apple improve things?
